Protective arrangement for electric power circuits



Sept. 17. 1940. Q R MASON 2,214,858

PROTECTIVE ARRANGEMENT FOR ELECTRIC POWER CIRCUITS Filed Feb. 23, 19:59

Inventor" I Charles F2. Mason,

His' btorney Patented Sept. 17, 1940 UNITED STATES PATENT OFFICEPROTECTIVE ARRANGEMENT FOR ELEC- TRIC POWER CIRCUITS New YorkApplication February 23, 1939, Serial No. 257,939

20 Claims.

My invention relates toimprovements in protective arrangements forelectric power circuits and more'particularlyto improvements inprotective arrangements oftthe pilot circuit control type disclosed inLetters Patent of the United States, 1,950,193, dated March 6, 1934, andassigned to the assignee of this invention. The general object of myinvention isto provide an improved pilot circuit control which iscontinuously supervised, is very flexible in application to systemsections regardless of the number of line terminals and parallelportions, requires but one source of current for energization, and pro--vides high speed discriminating protection.

In discriminating protective arrangements for effecting thedisconnection of a section of a [power line or system only on theoccurrence of a fault on the section by means of auxiliary controlcurrents over pilot circuits, any break in the pilot conductors, whichare sometimes rented from the telephone company, should make itself:known immediately. Otherwise the protective system may fail when it ismost needed. This is particularly the case where dependence is placed onthe continuity of the pilot circuit for tripping on internal faults. Inaccordance with my invention, I provide a continuously energized pilotcircuitso supervised as to makeknown substantially instantly. anyinterruption in the continuity of the circuit without, in any way,jeopardizing the line protection. Where sources oi direct current, suchas batteries at the stations, :are used to energize the pilot circuit,it is frequently disadvantageous to interconnect the batteries of twodifferent stations because of methods of groundingv the batteries andalso because trouble on one battery affects'the other. Moreover, the useof'a battery at each station is impossible when an attempt is made toprovide pilot wire protection for two or more parallel lines betweenstations. In accordance with my invention, I provide a pilot circuitprotective arrangement in which only one source of current is neeessaryfor energizing the pilot circuit. This source maybe a direct currentsource at the most convenient station location but alternating currentmay also be used. Further in accordance with my invention, I use a livepilot circuit for tripping on internal faults in order to obtain highspeed protection. Also in accordance with my invention I provide anextremely flexible protective arrangement which is readily and simplyextended to cover a line section even though more than two terminals areinvolved. These and other objects of my invention will appear in moredetail hereinafter.

This application is a continuation in part of my copending application,Serial No. 210,449, filed May 27, 1938, for Protective arrangement forelectric power circuits, the subject matter in said prior applicationbeing incorporated in this application together with additional subjectmatter.

My invention willbebetter understood from the following description whenconsidered in connection with the accompanying drawing, and its scopewill be pointed out in the appended claims.

In the accompanying drawing Fig. 1 illustrates diagrammatically anembodiment of my invention, shown single phase for simplicity, Fig. 2illustrates diagrammatically an embodiment of my invention asapplied toa three-phase grounded neutral circuit, the protective relay windingsbeing omitted for simplicity in understanding the invention, and Fig. 3illustrates diagrammatically a modification of the embodiment of myinvention shown in Fig. 2.

In Fig. 1, I have illustrated an embodiment of my invention'as appliedto a section B of an alternating current system. This section may be,for example, a power line extending between two stations shown as busses3 and 4. For simplicity in illustrating my invention, the power line isshown single phase. The section B is connected to the busses 3 and A bysuitable interrupting means which are illustrated as latched closedcircuit breakers 6 and 1 respectively. As illustrated, these areprovided with trip coils 8. The power line may include other sections Aand C suitably connected to the busses 3 and i respectively.

For controlling the circuit breakers 6 and I so as to disconnect thesection B from the system only when a fault occurs on this section, Iprovide in accordance with my invention a normally energized pilotcircuit comprising conductors 9 and i0 extending in general from one endof the section to the other. Further in accordance With my invention, Iso arrange the control of the circuitbreakers-S and I that only onesource of current, such as a battery H at station 4, is necessary forenergizing the pilot circuit. In order to detect any break orinterruption in the continuity of the pilot circuit, I provide at eachstation means, such as an auxiliary relay 12, whose Winding is in seriesrelation in the pilot circuit. This relay through its contacts l3, whichare open as long asthe pilot circuit is energized,

controls an alarm and indicating circuit I4 which consequently isenergized upon any interruption in the flow of current in the pilotcircuit.

In order that the pilot circuit may be continuously energized undernormal line conditions and yet be adapted to effect the opening of oneor both of the section circuit breakers under faulty conditions, Iarrange in accordance with my invention to control the pilot circuit bya circuit opening fault detector relay i5 at each station. The normallyclosed contacts of these relays are in series relation in the pilotcircuit. As shown, the fault detector relay I5 is a simple overcurrentrelay having its winding energized from a current transformer |6connected in the power line. However, as far as my invention isconcerned, any relay which will respond to a fault on the system,including the power line, can be used. Thus, as will be apparent tothose skilled in the art, the fault detector may be of the type whichresponds to voltage only, for example an undervoltage relay, or of thetype which responds to both current and voltage, for example a relay ofthe socalled distance type which responds to circuit impedance or acomponent thereof.

For controlling the circuit of the tripping coil 8 of the circuitbreaker at each station, I provide a circuit-closing tripping relay I],which in accordance with my invention is arranged to be energized onlyon the occurrence of predetermined faulty conditions of the circuit.These predetermined faulty conditions are internal faults, that is,faults on the section B under protection. In order to secure thisoperation, I arrange through suitable fault responsive means to connectthis relay in a circuit across that portion of the pilot circuit whichincludes the winding of the supervising relay I2 and the contacts of thefault detector l5. As illustrated, the fault responsive means forcontrolling the circuit of the tripping relay I! includes a second faultdetector 58 and a power directional responsive relay 2|. The secondfault detector I8 is also shown as a simple circuit closing overcurrentrelay but, as will be obvious to those skilled in the art, this relaymay be of the undervoltage type or the socalled distance or impedanceresponsive type. This second fault detector I8 has normally opencontacts I9 in the circuit of the winding of the tripping relay I? andmay also control normally open contacts 20 in the circuit of the circuitbreaker trip coil 8. The power directional relay 2| is illustratedsimply as comprising a current winding connected to be energized fromthe current transformer l5 and a potential winding connected to beenergized from a potential transformer 22. This relay may be of anysuitable type, examples of which are well known to the art. Thus it maybe an induction disk type of relay or the higher speed induction cuptype of relay. It may also include a well known voltage restraintfeature to prevent operation on a mere change in the direction of powerwithout an actu a1 fault condition. The power directional relay 2|controls normally open contacts 23 in the circuit of the tripping relayll and is so connected and arranged as to tend to close its contacts onpower flow from the bus into the section. At the station where thebattery II is located the circuit breaker trip coil 8 may be energizedfrom this battery, but at the other station the circuit breaker tripcoil 8 will be energized from the battery 21 at that station.

It will be observed that the relays I2 by virtue of their respectiveresistances, which are preferably substantially equal, constitutecurrent limiting means by which the normal current flow in the pilotcircuit is reduced to a satisfactory value. Obviously, if the relays I2are omitted, then other suitable current limiting means should beprovided. In order to insure substantially the same current flow in thepilot circuit under the different possible fault responsive operationsas under normal power'circuit conditions, the resistance of each of therelays Il may be made substantially the same as that of one of therelays I2. If this is done, then in every case the resistance of thepilot circuit is the same and includes the resistance of two of therelays |2, two of the relays ill, or one of each.

Under normal conditions, the parts are positioned, as shown in Fig. 1,and. the pilot circuit is energized from the source I I because thefault detector relays I5 have their contacts closed. The supervisingrelay I2 at each station is energized with its contacts I3 in thesupervising circuit I4 open. In case of a fault on the system of whichthe line section B is a part, the fault detector relay I5 at one or bothstations will open. Assuming that the fault is not on the section B butis fed through the section B, then both of the fault detectors l5 willoperate thereby deenergizing the pilot circuit and the supervising relayat each station to give an indication that the pilot circuit is open. Atthe station where power flows from the bus into the section B, the powerdirectional relay 2| will operate to close its contacts. Assuming thatpower flow is from section A. to section B and thence to section C, thefault detector I8 and the power directional relay 2| at station 3 willoperate to complete a circuit for the tripping relay II across thecontacts of the fault detector I5 and the supervising relay I2 atstation 3. However, this does not effect the energization of the pilotcircuit because at station 4, where power flow is from the section B tothe bus, the powered directional relay 2| keeps its contacts 23 open,and since the pilot circuit is also open at the contacts of the faultdetector relay |5, no current can flow in the pilot circuit to energizethe tripping relay I! at station 3. Moreover, no current can flow in thewinding of the tripping relay I! at station 4 because the contacts 23 ofthe power directional relay 2| at this station are open. Consequentlythe circuits of the trip coils 8 at the two stations remain deenergized,and the section B stays connected in the system.

Assiuning now an internal fault, that is to say, a fault on the sectionB which is fed from only one end, for example from bus 3 to section B.Under these conditions, the fault detector relay I5 at station 3 opensits contacts in the pilot circuit as before, but the fault detector I5at station 4 does not operate. Also the fault detector relay l3 and thepower directional relay 2| at station 3 operate to complete the circuitof the tripping relayI'I at this station. Since the pilot circuit is notopen at station 4, the tripping relay IT at station 3 is thus energizedto complete the circuit of the trip coil 8 of circuit breaker B. In thisway section B is disconnected from the system at station 3. At station 4neither the second fault detector I8 nor the power directional relay 2|what in advance of the fault detector relay I8. This may be accomplishedby giving the fault detector relay a more sensitive or lower settingthan that of the fault detector relay IB. Instead of using two separatefault detectors of different sensitivities, I may use a single faultdetector which controls two different contacts with differentsensitivities. Examples of such fault detectors are well known to theart. Un-

der some conditions, I may even Wish to use a single sensitivity faultdetector both to open the pilot circuit and also to close the circuit ofthe tripping relay.

In the event of a fault on section B fed from both busses 3 and 4, thefault detector I5 at each station operates, thus interrupting the pilotcircuit at two points and causing the dropout of the supervising relayI2 at each station and a suitable alarm thereby. Also, since power flownow at each station is from the bus into the section B, the powerdirectional relay 2| at each statiorr operates, and since there is faultcurrent indication at each station, the second fault detector I8 at eachstation operates. Consequently at each station the circuit of thetripping relay I1 is completed across the contacts of the fault detectorrelay I5 and the winding of the supervising relay I2 at the station. Thepilot circuit 9, I0 is accordingly completed at each end through thewinding of the tripping relay I! at the end, and the contacts 23 and I9of the power directional relay and the second fault detectorrespectively at that end. In consequence the tripping relay H at eachend is energized, and it, together with the second fault detector I8,completes the trip coil circuit at the end, thereby effecting thetripping of the circuit breaker at that end and the isolation of thesection B from the system.

In Fig. 2 I have illustrated my invention as applied to one end of asection B of a three phase grounded neutral power circuit. Thearrangement at the other end will be obvious by comparison with Fig. 1since there is no essential difference beyond the difference in thebattery connection to the trip coil. Since the connections of thewindings of the fault responsive relays will be obvious to those skilledin the art and are generally indicated in Fig, 1, they are omitted forsimplicity in understanding the control circuits. In the case of a threephase circuit, I provide three first, or sensitive, fault detectors I5I,I52 and I53 for faults between phase I conductors and a fourth alsosensitive fault detector I54 for faults between any conductor andground. These four fault detectors have their contacts in series witheach other in the pilot circuit. Across these series connected contactsand the winding of the supervising relay I2 and in the circuit of thetripping relay I! are the normally open parallel-connected contacts I9of the second, or high set, fault detectors I8I,

I82 and I83 which respond to faults between phase conductors on thesystem. In series with the contacts I9 are the contacts 23 of the powerdirectional relay 2I which may be a polyphase relay, examples of andconnections for which are well known to the art. In case single phasepower directional relays are used, the contacts which, in order toprovide preferential control for ground faults, may have normally closedcontacts I98 in series with the contacts I9 and 23 and normally opencontacts I9! in series with the contacts 23!) of a ground powerdirectional relay 2H3. The ground power directional relay 2H] respondsto the direction of flow of ground fault power in the section B at thestation 3. The contacts 23! and Bill of the ground fault responsiverelays ZIII and are connected in series across the series-connectedcontacts I9, 23 and I90 and in series with the Winding of the trippingrelay II. A series seal-in relay 25 may be arranged as shown to takeover the duties of the contacts of the high set fault detectors and thetripping relay. This relay, when energized, seals itself in to maintainthe trip coil circuit definitely closed regardless any change in thecondition of the contacts of the fault responsive relays and thetripping relay. In Fig. 2 the circuit breaker 6 is shown as equippedwith an auxiliary switch 26 of the a type, which opens when the circuitbreaker opens to interrupt the circuit of the trip coil and also todeenergize the seal-in relay 25. The operation of the embodiment of myinvention, shown in Fig. 2, will be apparent from the foregoingparticularly in connection with the explanation of the operation of theembodiment of my invention shown in Fig. 1.

If a power system includes parallel lines with a generating source atonly one end or a large source at one end and a relatively small sourceat the other end or if there may normally be large generation at bothends but under some conditions no generation at one end, then there maybe insuflicient fault current at one end to operate one of the high setfault detectors ISI, I82, I83, I84 at this one end. In this case thepilot circuit would be opened at the low current end of the section byone of the low set fault detectors I5I, I52, iii l but would not beclosed by a high set fault detector at this end. With the pilot circuitthus opened at this end, no tripping could occur at either end. Althoughthe probability of this happening may be greatly reduced by the use of adistance relay type of fault detector, I may, in accordance with myinvention, as shown in Fig. 3, substitute for the normally open contactsIS on the high set phase fault detectors ISI, H32, I83 and the normallyate a low set fault detector, the possibility of an open pilot circuit,which would prevent tripping on an internal fault fed from both ends, iseliminated. In other words, the energization of the tripping relay IT atthe high current end is assured in case of an internal fault even thoughthe high set fault detectors do not operate at the lower current end.Subsequently, tripping will occur at the other or low current end since,upon the opening of the circuit breaker in the faulty line at the highercurrent end, enough current will in general flow over the sound feederto sup plement the current supplied to the faulty feeder at the lowcurrent end to insure the operation of the high set fault detector. Evenin those cases where there is initially a relatively small input at oneend, this sequential tripping may be minimized by the use of a distancerelay type of fault detector.

It will be obvious to those skilled in the art that protectivearrangements embodying my invention are readily extended to apply tolines having more than two terminals since all that is necessary is tocontinue the pilot circuit from terminal to terminal and provide eachterminal with the protective equipment and control relays, shown in thedrawing, at each station.

While I have shown and described my invention in considerable detail, Ido not desire to be limited to the exact arrangements shown, but seek tocover in the appended claims all those modifications that fall withinthe true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. A protective arrangement for an electric power circuit having acircuit interrupter therein comprising a pilot circuit extending betweentwo points of the power circuit, a source of current for energizing saidpilot circuit, fault responsive means at each of said points jointlyoperative under normal conditions of the power circuit to maintain thepilot circuit continuously energized from said source and under faultyconditions of the power circuit to deenergize the pilot circuit,tripping means, and means for effecting the energization of the trippingmeans from the pilot circuit on the occurrence of predetermined faultyconditions of the power circuit.

2. In a protective arrangement for an electric power circuit having acircuit interrupter therein, means for controlling the opening of saidcircuit interrupter comprising a pilot circuit extending between twopoints of the power circuit, a source of current for energizing saidpilot circuit, means for tripping said circuit interrupter, a firstfault detector at each of said points, said first fault detectors beingjointly operative under normal conditions of the power circuit tomaintain the pilot circuit continuously energized from said source andunder faulty conditions of the power circuit to tend to effect thedeenergization of the pilot circuit, and means for replacing the pilotcircuit control of one of said first fault detectors and for energizingsaid tripping means from said pilot circuit on the occurrence ofpredetermined faulty conditions of the power circuit comprising a secondfault detector and a power directional relay.

3. In a protective arrangement for an electric power circuit having acircuit interrupter therein, means for controlling the opening of thecircuit interrupter comprising a pilot circuit extending between twopoints of the power circuit, a source of current at one of said pointsfor energizing said pilot circuit, a first fault detector at each ofsaid points having normally closed contacts in series relation in thepilot circuit operative on the occurrence of faults on the power circuitto open its contacts, a tripping relay having a winding, and meansresponsive to predetermined faulty conditions of the power circuit forconnecting said winding in the pilot circuit across the contacts of oneof said first fault detectors.

4. In a protective arrangement for an electric power circuit having acircuit interrupter therein, means for controlling the opening of saidcircuit interrupter comprising a pilot circuit extending between twopoints of the power circuit, a source of current for energizing saidpilot circuit, a first fault detector at each of said points said firstfault detectors being jointly operative under normal conditions of thepower circuit to maintain the pilot circuit continuously energized fromsaid source and under faulty conditions of the power circuit todeenergize the pilot circuit, a tripping relay having a winding, andmeans comprising a second fault detector and a power directional relayfor connecting said winding in the pilot circuit and for effecting theenergization of the pilot circuit on the occurrence of predeterminedfaulty conditions of the power circuit.

5. In a protective arrangement for an electric power circuit having acircuit interrupter therein, means for controlling the opening of saidcircuit interrupter comprising a pilot circuit extending between twopoints of the power circuit, a source of current for energizing saidpilot circuit, a first fault detector at each of said points said firstfault detectors being jointly operative under normal conditions of thepower circuit to maintain the pilot circuit continuously energized fromsaid source and under faulty conditions of the power circuit todeenergize the pilot circuit, a tripping relay having a winding, meanscomprising a second fault detector and a power directional relay forconnecting said winding in the pilot circuit and for effecting theenergizetion of the pilot circuit on the occurrence of predeterminedfaulty conditions of the power circuit, and means including a relayhaving a winding connected in the pilot circuit to be deenergized on anyinterruption of thepilot circuit including the operation of one of saidfirst fault detectors.

6. In a protective arrangement for an electric power circuit having acircuit interrupter therein, means for controlling the opening of thecircuit interrupter comprising a pilot circuit extending between twospaced points of the power circuit, a source of current at one of saidpoints for energizing said pilot circuit, a low set fault detector ateach of said points having normally closed contacts in series relationin said pilot circuit operative on the occurrence of faults on the powercircuit to open its contacts, a tripping relay having an energizingwinding, and means for connecting said winding in the pilot circuitacross the contacts of one of said first fault detectors on theoccurrence of predetermined faulty conditions of said power circuitwhereby to effect the tripping operation of said tripping relayincluding a high set fault detector and a power directional relay havingnormally open contacts in series relation with said winding.

'7. A protective arrangement for an electric power circuit having acircuit interrupter therein comprising a pilot circuit extending betweentwo points of the power circuit, a source of current for energizing saidpilot circuit, a first fault detector at each of said points said firstfault detectors being jointly operative under normal conditions of thepower circuit to maintain the pilot circuit continuously energized fromsaid source and under faulty conditions of the power circuit todeenergize the pilot circuit, tripping means, and means comprising asecond fault de tector and a power directional relay for effecting theenergization of the tripping means from the pilot circuit on theoccurrence of a fault on the power circuit between said two points.

8. In a protective arrangement for an electric power circuit having acircuit interrupter at each of two spaced points thereof, means forcontrolling the opening of said circuit interrupters comprising a pilotcircuit, a source of current for energizing said pilot circuit, a firstfault detector at each of said points said first fault detectors beingjointly operative under normal conditions of the power circuit tomaintain the pilot circuit continuously energized from said source andunder faulty conditions of the power circuit to -,deenergize thepilotcircuit, a tripping relay at each of said points, and meanscomprising a second fault detector and a power directional .relayat eachof said points for connecting the tripping relayv at the point in thepilot circuit to effect a tripping: operation of the tripping relay on;the, occurrence of predetermined faulty con- ,ditions of the powercircuit.

9. In a protectivearrangement for an electric power circuithaving acircuit interrupter therein, means for controlling the opening of saidcirat each of said points, each of said fault detec- .torsincludingnormally closed contacts in series relation in saidpilotcircuit, a tripping relay at each of said-points, and means comprising ahigh set fault detector and a power directional relay for completing acircuit across the contacts of 1 the low set fault detector at one pointand energizing the tripping relay at said point on the occurrence ofpredetermined faulty conditions of the power circuit.

. 10. In a protective arrangement for an electric :power circuit havinga circuit interrupter therein, means for controlling the opening of saidciro; cuit interrupters comprising a pilot circuit ex- 3 I a source ofcurrent at one of said points for energizing said pilot circuit anauxiliary relay havtending between-two points of the power circuit,

ing a winding connected in series relation in the pilot circuit, a lowset fault detector at each of said points, each of said fault detectorsincluding normally closed contacts in series relation in said pilotcircuit, a tripping relay at each of said points, and means comprising ahigh set fault detector and a power directional relay for completing acircuit across the contacts of the low set fault detector and thewinding of the auxiliary relay at one point and for effecting theenergization of the tripping relay at said point on the occurrence ofpredetermined faulty conditions of said power circuit.

1 1. In a protective arrangement for an electric power circuit having acircuit interrupter at each of two spaced points thereof, means forcontrolling the opening of said circuit interrupters comprising a pilotcircuit, a source of current for energizing said pilot circuit, anauxiliary relay at each of said points having a winding connected inseries relation with the pilot circuit, a first fault detector at eachof said points, said first fault detectors being jointly operative undernormal conditions of the power circuit to main tain the pilot circuitcontinuously energized from said source and under faulty conditions ofthe power circuit to deenergize the pilot circuit, a tripping relay ateach of said points, means comprising a second fault detector and apower directional relay for effecting the energization of the trippingrelay at the point from the pilot circuit and for maintaining theauxiliary relay deenergized on the occurrence of predetermined faultyconditions of the power circuit.

12. In a protective arrangement for an electric power circuit having acircuit interrupter at each of two spaced points thereof, means forcontrolling the opening of said circuit interrupters comprising a pilotcircuit, a source of current for energizing said pilot circuit, anauxiliary relay at each of said points having a winding connected inseries relation with the pilot circuit, a first fault detector at eachof said points, said first fault detectors being jointly operative undernormal conditions of said power circuit to maintain the pilot circuitcontinuously energized from said source and under faulty conditions ofthe power circuit to deenergize the pilot circuit, a

tripping relay at each of said points, means comprising a second faultdetector and a power directional relay at each of said points foreffecting the energization of the tripping relay at the point from thepilot circuit and for maintaining the auxiliary relay deenergized on theoccurrence of predetermined faulty conditions of the power circuit, andmeans controlled by the second fault detector and the tripping relay ateach point for effecting the opening of the circuit interrupter at thepoint.

13. In a protective arrangement for an electric circuit, a normallyenergized pilot circuit, means connected to the electric circuit foreffect- ,ing an interruption in said pilot circuit in re-.

sponse to faulty conditions of the electric circuit, a control devicehaving a winding, and means responsive to certain of said faultyconditions of the electric circuit for completing a circuit for saidwinding across the interruption in said pilot circuit.

14. In a protective arrangement for an electric circuit, a normallyenergized pilot circuit, means connected to the electric circuit foreffecting a plurality of interruptions in said pilot circuit in aresponse to predetermined faulty conditions of the electric circuit, aplurality of control devices each having a Winding, and means responsiveto one of said faulty conditions of the electric circuit forrespectively connecting the windings of said control devices across theinterruptions in said pilot circuit.

15. In a protective arrangement for an electric circuit, a single sourceof current, a pilot circuit normally energized from said source, meansconnected to the electric circuit for effecting one or moreinterruptions in the pilot circuit in response to faulty conditions ofthe electric circuit, a plurality of control devices each having awinding, and means responsive to that one of the faulty conditions whichproduces a single interruption in the pilot circuit for connecting thewinding of one of said devices across said interruption in series in thepilot circuit and to faults causing more than one interruption forconnecting the control device windings respectively across the differentinterruptions in series in the pilot circuit.

16. In a protective arrangement for an electric power circuit having acircuit interrupter at each of two spaced points thereof, means forcontrolling the opening of said circuit interrupters comprising a pilotcircuit, a source of current for energizing said pilot circuit, currentlimiting means at each of said points connected in series relation withthe pilot circuit, each of said current limiting means havingsubstantially the same current limiting effect, a fault detector at eachof said points, said fault detectors being jointly operative undernormal conditions of the power circuit to maintain the pilot circuitcontinuously energized from said source and under faulty conditions ofthe power circuit to deenergize the pilot circuit, a tripping relay ateach of said points, each of said tripping relays having substantiallythe same current limiting effect as each of said current limiting means,and fault responsive means at each of said points for comp cting acircuit including said tripping relay across the contacts of the faultdetector and the current limiting means at the point.

17. In a protective arrangement for an electric circuit, a normallyenergized pilot circuit, current limiting means connected in seriesrelation in said circuit, means connected to the elec tric circuit foreffecting an interruption in said pilot circuit in response to faultyconditions of the electric circuit, a control device having a Windingthe current limiting effect of said winding being substantially the sameas the current limiting effect of said current limiting means, and meansresponsive to certain of said faulty conditions of the electric circuitfor completing a circuit for said winding across the interruption insaid pilot circuit.

18. In a protective arrangement for an electric circuit, a source ofcurrent, a pilot circuit normally energized from said source, meansconnected to the electric circuit for effecting one or moreinterruptions in the pilot circuit in response to faulty conditions ofthe electric circuit, a plurality of control devices each hav ing aWinding, means responsive to that one of the faulty conditions whichproduces a single interruption in the pilot circuit for connecting thewinding of one of said devices across said interruption in series in thepilot circuit and to faults causing more than one interruption forconnecting the control device windings respectively across the differentinterruptions in series in the pilot circuit, and means for maintainingthe effective resistance of the pilot circuit substantially constantindependently of the number of interruptions in the pilot circuit.

19. In a protective arrangement for an electric power circuit, a pilotcircuit extending between two points of the electric circuit, a sourceof cur- 5 rent for energizing said pilot circuit, current limiting meansconnected in series relation in said pilot circuit, a control relay, andfault responsive means for effecting an interruption in said pilotcircuit and subsequently connecting'lfi said control relay across saidinterruption and said current limiting means for a predetermineddirection of power flow in the electric circuit.

20. In a protective arrangement for an electric power circuit having acircuit interrupter there- 15 in, means for controlling the opening ofsaid circuit interrupter comprising a pilot circuit extending betweentwo points of the power circuit,

a source of current for energizing said pilot circuit, current limitingmeans connected in series 20 relation in said pilot circuit and having apredetermined current limiting effect, a control relay havingsubstantially the same current limiting effect as said current limitingmeans, fault responsive means comprising a first fault detector foreffecting an interruption in said pilot circuit and subsequentlyconnecting said control relay across said interruption and said currentlimiting means, a second fault detector, and means controlled by saidcontrol relay and said' second fault detector for effecting the openingof said circuit interrupter.

CHARLES R. MASON.

